Orient played for 70 minutes with only 10 men to beat Peterborough and keep their automatic promotion hopes alive.
They went ahead after 14 minutes when Phil Bolland handled and Matt Lockwood scored from the spot.
Paul Connor saw red seven minutes later for elbowing Paul Carden, but Wayne Corden added a second for Orient on the hour with a superb drive.
Five minutes from time, Peterborough substitute Lloyd Opara ran clear to slot home a composed finish.
Leyton Orient: Garner, Zakuani, Lockwood, Tann (Barnard 70), Mackie, Simpson, Corden (McMahon 86), Easton, Keith, Steele (Alexander 76), Connor.
Subs Not Used: Morris, Ibehre.
Sent Off: Connor (21).
Booked: Steele, Alexander.
Goals: Lockwood 15 pen, Corden 70.
Peterborough: Tyler, Carden, Bolland (Opara 39), Arber, Farrell (Kennedy 79), Newton, Gain (Semple 73), St. Ledger, Logan, Crow, Ryan.
Subs Not Used: Harrison, Plummer.
Booked: Carden, Opara, Arber.
Goals: Opara 85.
Ref: P Melin (Surrey).